Gamma-rayConstraintsonElectromagneticCounterpartsofFastRadioBursts
Collaborators:TomonoriTotani,NoritaKawanaka
@Resuceu-RIKENJointSeminar201625July2016
ShotaroYamasaki(U-Tokyo)
“A blind search for prompt gamma-ray counterparts of fast radio bursts with Fermi-LAT data”
Yamasakietal.2016(arXiv:1604.03077,MNRASinpress)
Outline
1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork
TheDawnofFRBAstronomy(2007-)
• Fastradioburst(FRB)wasfirstdiscoveredbyLorimer(2007)throughpulsarsearch.FRB010724
• FRBsarefurtherconfirmedwithaconsiderablesample(17FRBsatthemoment,i.e.,Thorntonetal.2013;Keaneetal.2011&2016;Burke-Spolaor&Bannister2014;Spitleretal.2014&2016;;Championetal.2015;Ravietal.2015).
freq
uency
Time
Afewmsecduration
FastRadioBursts• Duration𝛿𝑡 ∼ afewmilliseconds
Indicatescompactobjects:
• Fluxdensity~ afewJyat~ GHzCoherentemissions:
• AnomalouslyhighDispersionMeasure(DM)(375-1500pc/cm3)PropagatingthroughtheionizedISM:
7FRB ~ ~ 3 1 cm0 ( / )r c t t msδ δ×
Evenat~kpc,TB isstillhigh(1012 K:upperlimitofastrophysicalsources)
-2
-4
Timedelay DMPulse width
ν
ν
∝ ×
∝
Comological origin?(DM-bplot)Thorntonetal.2013
Pulsars
FRBs
Latitude
DM
HighDMsatrelativelyhighlatitudes
FastRadioBursts• CosmologicalDistances
HighDMvaluessuggest→ indicatingcosmoloicalorigin(ifdilationoccursbypropagationthroughISM)
• EventrateØ EventrateofGRBs:Ø Eventrateofcore-collapseSNe:
• Nocounterpartssofarinotherwavelength(exceptforFRB150428)
• ConstraintsonprompthighenergyemissionØ 10-100keV:(Tendulkar+16,Murase+16)Ø 1-100GeV:(Yamasaki+16,thiswork)
3 1 14 ~1010 / sky / day or gal yr− − −:6 1 1gal r~10 y− − −
2 1 1gal r~10 y− − −
Recentobservations(Totani-san’stalk)
• Keane+16Ø ATCAdetected~6dayradioafterglowassociatedwithFRB150428Ø AhostgalaxywasfoundinSubarui’bandimageforthefirsttimeØ ThehostgalaxyofFRB150428iselliptical(hencenotstar-forming)
→progenitor=oldstellarpopulation(i.e.NS-NSmerger,etc..)
• Spitler+16&Scholtz+16Ø AreciboFRB(FRB121102)repeats17timestotalØ AllDMsconsistentwithasinglevalue(~559pc/cm3)→prgenitor=super-giantpulsesfromyoungpulsarsormagnetars?
Ø NopromptX-rayemissiondetected
→ AllFRBsrepeat?Atleasttwopopulations?
…butcurrently,itsassociationunderdebate(AGNactivity?)
LeadingProgenitorModels
• Pulsaremissions• SGR(Magnetar)flares• AsteroidscollisionwithNS
• NS-NS(BH)merger• HMNScollapse• WD-WDmerger• BH-BHmerger
(ImageCredit:NASA/CXC/SAO) (Rezzollaetal.2011)
Catastrophic(One-off)Repeatable
Possiblesiteofgamma-rayemissions
FRB121102(Arecibo/GBT) FRB150428(Parkes)
Outline
1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork
Motivations
• Whatevertheprogentoris,itisreasonabletoexpectcorrespondingms-durationhigh-energyradiationsattheonsetofFRBs
• NoassociationwithX-ray(Swift-BAT)andgamma-ray(Fermi-GBM)emissiondetectedyet
→UseFermi-LATPass7Rep (1-100GeV,2008-2015)all-skydata
• RadiotelescopesfindingFRBsatcosmologicaldistanceshavenarrowerfield-of-views(∼ 15arcminbeamsizeforParkes)thanFermi-LAT
→RatherthanexaminingtheFermi-LATdataatknownFRBlocations(z=0.5-1.0),wesearchforms-durationgamma-rayflashesfromnearbypotentialFRBs(z<0.02)
Fermi-LAT(LargeAreaTelescope)
• Effectivearea:8000cm2
• EnergyRange:20MeV-300GeV• Angularresolution:0.8deg @1GeV• Pair-conversiontelescope(γ→e+ +e- )
Resolvedgamma-raysky(0.1-300GeV) Widefieldofview(~2.4sr)
(ImageCredit:NASA)
SchematicPicture
Gamma-raytelescopeFOV(wide&shallow)
RadiodetectedFRB(z=0.5- 1.0)
NearbypotentialFRBs
Parkes(Radio)FOV(narrow&deep)
Previousstudy(Tendulkar+16) Thiswork(Yamasaki+16)
observer observer
→ Weimprovethedetectionsensitivitybyafactorof≳100
Sensitivity line
Data&SearchAlgorithm
• Timespan:6.8yearsfromJuly31,2008(UTC)toJune18,2015(UTC),thetotalamountofgoodtimeintervalsis79%ofthis.
• Photondata:“P7SOURCE”(smallercontamination)
• EnergyRange:1-100GeV(Initiallyweused30MeV-300GeVwhichturnedouttoincludelargebackgroudcontamination)
• Searchtimewindow:∆t=1,2,5,and10msaretried.
• Weconsiderareferenceγ-rayevent,andotherγ-rayeventsaresearchedwithin2◦ radiusfromthereferenceinthetimeintervalof∆tstartingfromthereferenceevent.
Outline
1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork
Results
• ~100flashcandidatesaredetected
• Aretheygenuine?orfalseevent?
→Weexaminedthesignificanceoftheseevents
TimeWindows
FalseEventsfrom brightpointsources
> 4 x 10-9 phs/cm2/s 𝜃 < 2.0 deg
#1.Thephotonfluxdistributionofthenearestpointsourcetoeachevent
#2.Angularseparationdistributionfromeacheventtothenearestpointsource
CUT CUTEventsoriginatingfromneabypoint sourcesEventsoriginating from
brightpointsources
FalseEventsfromdiffusebackground
• Afterremovingeventsfrombrightsteadypointsources,wefoundnoflasheventsat|b|>20°
• Theexpectednumberofeventsfromthediffusebackground(black)canaccountfortheremainingflasheventsat|b|<20° (grey)
• Weusedthisfinalresult(noganmma-rayflasheventsat|b|>20°,remainingskyfraction~0.68) tosetanupper-limitonthefluxofms-scalegamma-rayflashes #Galacticlatitudedistributionofremainingevents
(Nphoton =2,Δ𝑡 =10ms)
黒:Diffuse BG からの理論値
非検出 非検出Non-detection Non-detection
Outline
1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork
Upper-limitongamma-ray/radiofluxratio• TheStandardcandleapproximation:wedefinegamma-raytoradioluminosity
ratiointherest-frameofanFRBas
• WeassumethatξisthesameforallFRBs,andthatFRBshaveapower-lawgamma-rayspectrumwithaphotonspectralindexΓ(herewetakeΓ=2)
• Themaximumredshiftofdetection()isdeterminedby:
• VolumetricFRBrateevolution isincludedas
• Theupper-limit onξ(2-sigma)isderivedforβ=0-4bysolving
w/
Duration(3ms)Numberofobserved
γ-rayflashes (< 2)withinΔt Differentialγ-rayfluxofanFRBEffectiveareaofLAT
(ξ:thekeyparameterinouranalysis)
NumberofFRBsinz<zmax
Exposuretime(fori-thtimebin)
Direction(forj-thskygrid)
Numberofdetectableγ-rayflashesfromFRBs(fori-thtimebinandj-thsky direction)
Result
ξ≲ 4.2 − 12 ×103 (@GeVband,thiswork)Ourlimitonξisstrongerthanthatobtainedfromnon-detectionofradio-detectedFRBsbyFermi-LAT(typicallyξ≲ 4×1045)
Themulti-wavelengthspectraoftypicalgamma-raypulsars
Thompson, D.J.2004a,inIAUSymp.218
• Typicallyξ =106 – 108
• Someyoungpulsarsexhibitξ <104
• ξofpulsarsaredistributedinawiderange(ξPSR =104 –108),andourresult(ξ≲ 106)doesnotexcludeapossibilitythatFRBshaveasimilarGeVgamma-raytoradiofluxratiotopulsars
SGR1806-20(magnetar)flaresinDec2004Initialflare
(IF)
Pulsating tails(PT)
TheSEDofInitialflare
~10– 100keVobservationonly
• Thebrightestgamma-rayeventbutnon-detection intheradioband:
(𝜈𝐹𝜈);<=>?< 1.1MJy(Tendulkar+16)
• Gamma-ray:~keVobservationonly→WeestimatetheGeV fluxofIF
usingPT’sSED:
(𝜈𝐹𝜈)A<;=BC~0.51 ergcm-2 s-1
• Thelower-limitonξforSGRisderivedas:ξSGR≳ 103.E
• Thisismarginallyconsistentwithourlimit: ξ≲ 4.2− 12 ×103
Outline
1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork
Summary• Wesearchedms-scalegamma-rayflashesinthe7-yrFermi-LATdata,
motivatedbythepossiblegamma-raycounterparttoFRBs– About100flashcandidatesaredetected
– Aftercarefuleventcuts,theremainingeventsatlowlat.regions(|b|< 20°)areconsistentwiththestatisticalflukesoriginating fromdiffusegamma-raybackground
– Noms-scale(∆t<10ms)gamma-rayflashesfoundathigh lat.regions(|b|>20°, skyfraction~0.68)
• Consideringthez-evolutionofFRBratedensity,wesetupperboundsonGeVgamma-raytoradiofluxratio(≡ ξ)ofFRBs:ξ≲ 4.2 − 12 ×103
– ξofpulsars:ξPSR =104 –108→ wecannotexcludethatFRBshavesimilarvaluesofξtopulsarsingeneral
– ξofamagnetar(SGR1806-20):ξSGR≳ 103.E→ theupperboundontheradiofluxforSGR1806-20ismarginallyconsistentwithourlimitofξ
Outline
1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork
TestingbinaryNSmergermodelforFRBs• BinaryNSmergersarepossibleoriginofFRBs(e.g.Totani2013,
Zhang+2016,Wang+2016)
• Theemissionmechanismproducingcoherentradioemissionisunclear(Magneticdipolaremission,curvatureradiation,synchrotron,InverseCompton…)
• Theoreticalstudiespredictpossibleelectromagneticemissionsduringinsipralphase(priortomerger)(e.g.Hansen&Lyutikov2000,Lai2012,Wang+2016)
• UsingbinaryNSmergersimulationdata(K.Kiuchi,M.Shibata),wetestproposedscenariosandconsiderarealisticmodelforFRBsifpossible
t=tmrg – 1.4ms(i=22)
t=tmrg – 0.7ms(i=23)
H4EOS,1.35MSUN – 1.35MSUN
t=tmrg(i=24)
t=tmrg +0.7ms(i=25)
H4EOS,1.35MSUN – 1.35MSUN
Angularvelocityevolution(beforemergertmrg- 4.2ms<t<tmrg)
V =R×Ω
Theangularvelocityenhancedbyafactorof~3-10inthelast0.7msbeforemerger
• HowenergyisextractedfromthespinandorbitalmotionofinspiralNS?
• Iscoherentemissionpossible?
• Themagnetosphericenvironment(density,opacity…)
Openquestions:
(Totani2013)